Make alias symlinks relative

[chupakobra6]

Summary

• create alias symlinks with targets relative to each alias parent directory
• preserve nested aliases when the simulation storage root is moved
• limit the change to newly created aliases; existing absolute links are not migrated

Testing

• python -m ruff format --check
• python -m ruff check
• python -m ty check src
• full pytest suite on Python 3.13: 168 passed, 4 xfailed, 1 xpassed
• focused regression test on Python 3.11, 3.12, and 3.13

Closes #54